1. Technical Field
The present invention relates to printed circuit boards, and more particularly relates to an improved solder mask design for the surface of a printed circuit board.
2. Related Art
Printed circuit boards (PCB's) are often fabricated with a solder mask on the top surface. The solder mask is generally a dielectric material that sits over the surface of the PCB, and includes openings for surface lands and the like. The solder mask provides a protective barrier between surface lands on the surface of the PCB in order to prevent the inadvertent flow of solder between adjacent surface lands during a heating process. The inadvertent flow of solder could occur, for example, when the board is subjected to heat in order to attach components to surface lands. The solder mask prevents solder from running between surface lands causing inadvertent short circuits (i.e., solder bridging). Additionally, by placing the solder mask over a wire, the solder mask prevents unwanted "capillary action" of the solder, i.e., it prevents the solder from running along the wire down into a plated through hole (PTH).
The solder mask is generally comprised of a clear epoxy material such as a photosensitive dielectric that can be placed on the PCB either as sheet of material or as a liquid coating applied with a curtain process. The applied material may then be selectively cured by exposing predetermined areas of the material to radiation to define a final solder mask. A radiation mask is typically used to block portions of the radiation thereby masking those areas of the PCB where openings in the solder mask are desired. After the PCB is selectively exposed to the radiation, a final cleaning step can be used to remove the uncured areas of the solder mask (i.e., where openings will exist).
Because of the close spacing of surface lands and wires on a PCB, the imprecise placement and tolerances of openings in the solder mask on the PCB can cause failures. In order to avoid failures, a minimum amount of coverage of the solder mask must be maintained between closely spaced surface lands. In addition, sufficient clearances must be maintained around PTH's in order to ensure that the solder mask material is not drawn into the PTH. This type of problem is precipitated during the heating process, when portions of the solder mask can be inadvertently drawn into a PTH and attach to the copper plating on the walls of the PTH. When such a dielectric material attaches to the copper plating, reservoirs can be created that attract chemicals from a subsequent processing step thereby causing the PCB to fail.
Referring to FIG. 2, a solder mask design 46, such as that used for a ball grid array is depicted on a surface of a PCB with four surface lands 30, 32, 34 and 36. FIG. 2 also depicts a PTH 40 centered in a PTH land 38, along with a dog bone wire 42 connecting PTH 40 to surface land 36 and forming what is generally referred to as a dog bone structure. Solder mask 46 includes circular openings defined by dotted line regions 44 that surround each of the surface lands 30, 32, 34, 36, and region 45 that overlays PTH land 38. In order to provide maximum utility, a barrier having no less than a minimum width (e.g., 4 mils) as indicated by arrow 50, must be maintained between the region 45 that surrounds PTH 40 and the dotted line region 44 that surrounds adjacent surface land 32 in order to ensure the mechanical and functional integrity of the solder mask 46. At the same time, the largest possible clearance, as indicated by arrow 51, must be maintained between the PTH 40 and solder mask region 45 in order to prevent the solder mask material from entering PTH 40. Unfortunately, neither the barrier width 50, nor the clearance 51 can be enlarged without affecting the performance of the other. Because the barrier width 50 must be no less than some predetermined minimum distance, clearance 51 around PTH 40 is often not as large as desired, resulting in a potentially defective PCB.
Therefore, without some way of better ensuring that the dielectric material from the solder mask does not inadvertently get into the PTH's, failures will continue to occur.